What does "Depth of Field" mean?

Bruce Taylor's Konvas 2M

Depth of Field, also called DOF, is the distance in front of and behind a certain object that is considered imperceptably in focus. The shallower the DOF, the more objects closer or further from the subject will appear out of focus. This can aid filmmakers in telling a story with selective focus, which helps to keep the audience's attention on who or what the director wants (which is the subject in focus).

A combination of several things create shallower or longer DOF: the width of the aperture (calculated in f-stop settings), the recording target size (or focal point size), the lens focal length (a 35mm lens versus a 75mm lens), and the distance of the subject from the camera (the focal distance).